In air brake systems, such as those implemented in commercial trucks, compressed air from the vehicle compressor is dried prior to delivery to the vehicle brakes and other compressed air operated components. The compressed air is dried to make the vehicle brakes operate effectively, and also to prevent damage to the brakes and other downstream components. Typically, an air dryer with a desiccant bed is placed between the compressor and the brakes. The desiccant bed air dryer removes moisture from the compressed air thereby permitting only dried compressed air to continue downstream to the brakes and other components.
Several problems exist with desiccant bed air dryers. First, the desiccant degrades as it becomes contaminated with organic components, such as oil vapor or aerosols, for example. As such, the desiccant bed must be periodically changed in order to maintain an effective air dryer. Second, desiccant beads have a saturation point, beyond which no additional moisture can be absorbed. In order to maintain an effective desiccant air dryer, a periodic, purge of the desiccant is required in order to keep the desiccant from becoming saturated. In typical desiccant bed air dryers, a purge cycle is established when the compressor unloads, thereby allowing the desiccant to regenerate. In some situations, the compressor is required to maintain a lengthy cycle before it can be unloaded. These situations arc somewhat frequent in commercial vehicles, such as during rapid braking and use of ancillary air powered equipment. During these long compressor cycles, the compressor charging duration can exceed the desiccant water absorption capacity and subsequent wet air can pass downstream to the brake system, or other components, where it can cause damage or other problems.
While it is known that membrane air dryers can produce a steady stream of dried air without requiring periodic change or periodic purging, membrane air dryers have generally only been implemented in clean air systems. Contaminants, such as oil aerosols, hinder the performance of membrane air dryers since the hydrophilic membrane tubes are resistant to organic components. The organic contaminants enter the membrane and foul up the surface of the membrane. This fouling of the membrane by organic contaminants prohibits air from passing through the membrane, thereby reducing the effectiveness of the membrane air dryer. As such, membrane air dryers have not been used in connection with vehicles.